Biomechanics in Orthodontics Anchorage Considerations-A Review
Biomechanics in Orthodontics Anchorage Considerations-A Review
Biomechanics in Orthodontics Anchorage Considerations-A Review
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Received date: April 3, 2019, Accepted date: April 17, 2019, Published date: April 19, 2019
Copyright: ©2019 Kulshrestha R. This is an open-access article distributed under the terms of the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original
author and source are credited.
*Corresponding Author: Rohit Kulshrestha, Terna Dental College Nerul Navi, India, Tel. No: +919870499761;
Email: kulrohit@gmail.com
Abstract
To anchor is to secure firmly, to hold an object against movement; anchorage is that which provides the secure hold.
Specifically, orthodontic anchorage is the ability to prevent tooth movement of one group of teeth while moving another
tooth or teeth. The nature of tooth movement depends on the ratio of the applied moment relative to the applied force
(M/F ratio) at the orthodontic bracket. The way a tooth moves is dependent on the force and moments applied on the
bracket (via elastic, coil, loop, etc.), and the actual force distribution about the periodontium (stress-strain relationship).
The force distribution is a function of the tooth’s center of rotation. The aim of this article was to review the different
mechanics seen and used during anchorage preparation.
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The Effect of a High M/F Ratio Applied to the Anchor Figure 2: Tipping produces more movement at the
TeethPerspectiveIntroduction
crown or occlusal plane compared with translation. For both
An applied force at the crown produces uncontrolled teeth in this illustration, the center of resistance of the tooth is
tipping as a result of the moment of the force. The applied displaced the same distance.
moment (moment of the couple) counteracts the tipping effect
of the force. The applied moment acts in the opposite direction
of the moment of the force. It moves the root(s) toward the
extraction space. In addition, as the magnitude of the applied
couple increases, the rotation of the tooth would move the
crown away from the space (Figure 1) [2].
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The application of unequal moments must also satisfy controlled space closure. The core principle of loop design for
Newton’s laws. Because the moments on each end of the spring all these springs is increased stiffness of the wire on the
are unequal, the total force system must have additional effects. anchorage side of the spring [5]. All conversely, asymmetric
Vertical forces, intrusive to the anterior and extrusive to the positioning of the loop towards the anchorage teeth has a similar
posterior, are also acting. (The vertical force magnitude depends effect because wire stiffness is inversely related to the third
on the difference in the 2 moments and the distance between power of the length, an off-centered or asymmetrically
anterior and posterior attachment points) (Figure 4). It also positioned spring will deliver greater moments to the teeth that
results in occlusal plane discrepancies between the anterior and it approximates (Figure 5) [6].
posterior teeth. The posterior teeth may be positioned with the
crowns distally tipped and the roots mesially oriented [3].
Clinical Techniques Using Differential Moments for Another method of anterior retraction that uses a
Anchorage Control differential moment strategy for anchorage control is combined
incisor intrusion and retraction. This simple yet effective
The data were collected, summarized, and coded. All the appliance uses the tip back moment of the intrusion arch for
statistical analyses were performed using the Statistical Package creating the large posterior M/F ratio. The retraction force is
for Social Sciences (BM Corp. Released 2013. IBM SPSS applied with either coil springs or elastic chain (Figure 6). By
Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp). carefully controlling the intrusive and retraction forces, the
The following statistical procedures were performed: the overbite and over jet can be simultaneously corrected. Careful
construction of frequency distribution tables, one-way analysis monitoring is crucial for successful anchorage control during
of variance (ANOVA), and post-hoc tests. P ≤ 0.05 was space closure. A frequently overlooked consideration in
considered statistically significant. anchorage control the first-order side effects of space closure.
The mesially directed, buccally located force on the molar will
The T-loop spring described by Burstone and subsequently tend to produce a mesially inward rotation. This rotation gives
refined or modified as a simple yet effective device for the appearance of anchorage loss when viewed from the buccal
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perspective. However, distalization of the molar may not result of the moment applied to achieve anterior root correction.
be necessary; a mesially-outward first-order rotation may be all The anterior teeth are extruded while the posterior teeth
that is needed for regaining the desired molar position. A experience an intrusive force.
transpalatal arch provides an excellent means for preventing this
Anchorage requirements are critical during root correction
side effect or actively correcting it [7].
of the anterior teeth. Rowboat effect, moving the entire
Figure 6: Combined anterior retraction and deep overbite maxillary dental arch forward and resulting in a more Class II
dental relationship, may be observed as the roots of the anterior
teeth are moved lingually. The use of headgear to support
anchorage during anterior root correction may help to minimize
or eliminate this side effect. The root spring used for anterior
root correction may be fabricated using 0.022 × 0.016 inch
(ribbon wise) titanium molybdenum alloy (TMA), or 0.021 ×
0.025-inch (edgewise) TMA, to be inserted into 0.022 × 0.028-
inch edgewise brackets. The root spring is placed into the
brackets of the anterior teeth, stepped up around the canines and
premolars, and extended distally as a cantilever with hooks
mesial to the first molars (Figure 7) [9].
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Pre-activation bends are placed at the gingival position of the Figure 10: A. Occipital-pull headgear. B. Face bow typically
anterior step up and a gentle curvature is incorporated bilaterally used with occipital pull.
along the posterior cantilever (Figure 9). The amount of force is
measured on the right and left sides and trial activation is made
on each side of the spring [10].
Figure 9: Frontal view of an anterior root correction spring Figure 11: Resolving a force into its components along axes of
with bilateral activations (A). Once activated, the distal arms of interest.
the root spring will be pulled occlusally and hooked on the In cases where combination head gears are used vector
bypass arch wire (B). addition is accomplished by resolving the force along its line of
action into its components along the horizontal and vertical axis
Biomechanics of Headgear in Class II Dental and
as shown in Figure 11. An example of doing so is shown for
Skeletal Corrections
Class II elastic force to the maxillary arch in Figure 12.
Biomechanics
A headgear can deliver only a net single, simple force. To
determine the effects of headgear force, one merely needs to
examine the line of action of the force with respect to the body
to which it is applied-e.g., tooth, arch, or maxilla. Figure 10A
shows an occipital pull headgear in place. Figure 10B
demonstrates that the strap’s pull-the force’s line of action is
well above the center of resistance of the maxillary first molar.
At the maxillary first molars center of resistance, the headgear
force system has a distal component, an apically directed
vertical component, and a large root-distal movement [11].
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1. Anchorage control.
Anchorage Control
In Class II extraction treatment, to ensure that buccal segments of teeth do not move mesially when anteriors are retracted. In a
general sense, headgear force is used to control the side effects of intra oral mechanics which results in eruption of teeth. An eruptive
force applied to a molar is shown in Figure 14A. Such side effects are seldom desired.
Figure 14. A. Vertical force on molar tube, a side effect from intraoral mechanics.
B. Vertical component of occipital headgear force negates resting position. One needs to see the angle and level of the final
extrusive intraoral force side effect. line of action after the strap forces have been applied to know
exactly the force of the headgear system.
The side effect force tends to extrude the molar, and the
moment of the force-expressed at the center of resistance of the Tooth Movement
tooth-produces a root-buccal, crown lingual moment tending to
tip the molar crown into lingual cross bite. Applying an occipital
headgear force, whose line of action is shown in Figure 14B, In class II patients, if one adjusts the level of the outer bow
produces a vertical intrusive component of force that negates the such that a horizontal force is produced that passes through the
vertical extrusive force of the side effects even though the head center of resistance of the maxillary first molar, and the patient
gear force is not applied continuously. This vertical force is of a wears the headgear at the level of 14 hours each night
much higher force level than the force of the side effect. The consistently, than the molars will move distally 2 mm in 24
line of action is determined after the headgear strap has been months without tipping [14]. If the line of action of the headgear
applied to the outer bow, and the outer bow has deflected to its force is adjusted so that there is a vertical component tending to
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intrude the molar, as shown in Figure 14B, the headgear If the headgear force is applied through the center of
forces tend to prevent extrusion from intraoral side effects. If the resistance of maxilla (apical level between the premolars) and a
line of action of headgear force has an extrusive vertical preadolescent patient wears the headgear at least 12 hours each
component, the molar will extrude, independent of the night (at least 14 hours each night for adolescent patients) the
individual patient’s skeletal pattern, unless there is a large forward component of the maxillary growth is redirected [15].
intrusion force from the arch wire on the molar. This situation is Occasionally, especially in patients who wear the headgear at
now usual in intraoral mechanics. the level of 16 hours a night, the redirection is in a posterior
direction. The total magnitude of growth does not change but its
Orthopedic Changes direction is changed.
Figure 15. A: Force through the center of resistance of the maxilla. B. Typical redirection of the maxillary growth at ANS as
seen on cranial base superimpositions.
Seven sequential steps may be used in logical sequence to design the headgear force system for any orthodontic application. They are:
1. Determine the center of resistance of body to which headgear force is being applied, whether tooth or segment or arch or maxilla.
2. Then determine the force system through the center of resistance that will produce the changes desired. One thinks of the force and
moment at the center of resistance (Figure 15).
(a) Horizontally.
(b) Vertically.
(d) How far from the center of resistance, should the force be applied?
As an example (Figure 16), if one wants to steepen an occlusal plane, erupt the unit, the prevent mesial movement of the unit. A unit
could be a tooth, segment, arch or maxilla. A cervical headgear with a low outer bow generates a large moment about the center of
resistance that will tend to steepen the occlusal plane. The vertical component of the headgear forces acting at the center of resistance
will erupt the unit. The distal component will tip the unit distally.
(b) Cervical.
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Figure 16. Example of a commonly used force system: cervical headgear with low outer bow. Headgear force is shaded. The
equivalent force system at the center or resistance is in black.
5. Bend the outer bow angulation and adjust its length to deliver the desired line of action after the strap force is applied.
(a) In rotating the unit-that is, using a large moment in comparison to the applied force through the center of resistance-one
should use low strap forces to avoid high local stress in the periodontal ligament (150-200 g per side).
(b) If the line of action of headgear passes close to or through the units center of resistance, 400 to 500 g per side can be used.
7. Monitor for changes as treatment proceeds. Adjust the force line of action and force magnitude as necessary.
Figure 17: Occipital-pull headgear at the level of 12 hours per night (10 hours per night for conscientious adults) to control side
effects from maxillary incisor intrusion via a base arch.
An example of occipital headgear to control the occlusal plane and prevent side effects from maxillary incisor intrusion is given
in Figure 17. The headgear force is applied well away from the molars center of resistance to generate a large counter clockwise
moment, negating the clockwise moment for the intrusion arch. The outer bow is bent high and is cut short to provide the desired line
of action. If using a 0.016 inch stainless steel intrusion arch, 60 g will be generated at the midline if the wire is activated 90° just
mesial to the first molar tubes when a single helix is placed in that position. if the patient has a maximum anchorage class II
malocclusion, the angulation of the outer bow can be lowered and the headgear force increased to 400-500 g per side. The result will
be a larger horizontal distal component of force [16].
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Cervical pull
Figure 19 shows three possibilities for applying cervical
pull to a maxillary unit. The example at the top of the figure has
the outer bow low. The equivalent force system at the unit’s
center of resistance has an extrusive component, a distal
component, and a large moment that trends to steepen the
occlusal plane [17].
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Asymmetric Headgear It is usually best to stop using this mechanism if cross bite
development begins to occur [19].
Droschl H. The effect of heavy orthopedic forces on the maxilla in growing Saimiri sciureus
Baumrind S, Korn EL, Isaacson RJ, West EE, Molthen R. Quantitative analysis of the
Sangcharearn Y, Ho C. Maxillary incisor angulation and its effect on molar relationships. Angle
Carlsson R, Ronnermann A. Crown root angles of upper central incisors. Am J Orthod 1973; 64:
147-154.
Proffit WR, Fields HW, Ackerman JL, Sinclair PM, Thomas PM, Tulloch JFC. Contemporary
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